Voltage control of power transformers and inductance control of reactors used during electric power transmission are normally carried out with the aid of an on-load tap changer which makes it possible to change between different terminals of a winding. This is to be done under loading, and for that reason the tap changer must be capable of handling both the voltage difference between the terminals and the current through the winding. To reduce the intensity of the unavoidable arcs which arise during the connection operation, usually also resistors are connected during the connection operation. Depending on the location of the power transformer or the reactor in the electric network, the number of connection operations of a tap changer during the technical life of the transformer or the reactor may amount to several hundred thousand. A tap changer has a complicated mechanical structure which, in combination with the great number of use cycles under mechanical loading, makes the tap changer one of the most exposed components of a power transformer or a reactor.
The tap changer is exposed to wear by mechanical abrasion and arcs. This can manifest itself in a slow and gradual change of the performance of the tap changer; for example, the connection time may change because of changes in the dimensions and mutual distances of components included, such as contacts and other movable parts. The change process may at a certain stage accelerate and lead to the tap changer being incapable of functioning. This may, among other things, cause costly power failures.
To prevent functional incapability of a tap changer, the tap changer is checked. One way of doing this is an inspection of the tap changer, which requires opening of the transformer/reactor. This causes a lengthy service interruption. This is therefore carried out as seldom as possible, with an ensuing risk that a significant deterioration may occur before the next inspection.
It is known to continuously monitor the temperature of components of the tap changer, where the wear leads to an increased temperature, for example in contacts which will have increased contact resistance because of carbon deposits originating from arcs in the transformer oil. An increased temperature is then a warning and alarm signal which gives cause to inspection of the tap changer. However, significant heating only occurs in some of the change processes of a tap changer, and then only after an advanced ageing process.
Another known method is to continuously monitor the power consumption of the electric motor which drives the mechanics of the tap changer. Wear processes which manifest themselves in increased friction lead to an increased power consumption during the connection operation which can then be used as a warning or alarm signal. As in the case of temperature monitoring, this monitoring is limited to only some of the possible wear processes.
The connection operation generates sound which may be analyzed with respect to changes in the tap changer. Changed distances, changed friction etc., as a consequence of wear, influence the sound from the connection operation. Japanese patent document A 6-13248 describes a technique whereby the sound from a connection operation, which is divided into a number of sound pulses related to part-processes, is analyzed with respect to time differences between the beginning of these pulses. If one or more of these time differences deviates too much from values determined in advanced, a warning or alarm signal is released. U.S. Pat. No. 4,159,446 describes a system for detecting, by analog signal processing of a signal envelope from a connection operation, incorrectly adjusted contacts by analysis of the signal envelope, preferably by a skilled person, with respect to time differences between signal peaks in the signal envelope. With the methods described in Japanese patent document A 6-13248 and in U.S. Pat No. 4,159,446, there is a risk that not all changes in the tap changer manifest themselves in changed time differences between the sound pulses and that only heavy wear results in significant changes of these time differences.
A complete analysis of all sound from a tap changer at the time of each connection requires such calculating and memory resources that it is not practically feasible.